1. Field of the Invention
This invention relates to electrostatic printers and in particular, to an electrostatic print head which utilizes a stylus of unique cross section which reduces background noise produced on the recording medium over that produced by a stylus of the prior art.
2. Prior Art
Electrostatic printers are well known. Such printers are described for example in U.S. Pat. Nos. 4,672,399, 4,672,400 and 4,731,542. Electrostatic printers use print heads containing electrically-conductive styli. Typically, these styli are round in cross section, placed close together and separated by an insulative material. When a medium passes by the styli and a voltage is applied to selected styli, electrostatic charge is deposited on the medium adjacent the selected styli such that the medium will subsequently attract and hold a toner and thereby make the charge images visible. This mechanism is well known in the art. While styli with rectangular cross section have been disclosed, to ensure that the print on the medium is dense and further to ensure that the medium can be printed with an optimum speed for the resulting desired image quality, rectangular styli have usually been selected with an aspect ratio that is made nearly one to one (i.e., the ratio of width to length of the cross section is approximately one to one). Rectangular styli whose thickness is much less than their width have also been used. See, for example, U.S. Pat. No. 4,521,790 entitled "Electrostatic Printer of Video Pictures with Grey Tones" which discloses use of a writing head having electrodes with a "Substantially elongated cross-sectional geometry such that a width dimension is significantly less than a length dimension." (Abstract, lines 4-6.) Unfortunately, styli of the '790 patent when used in a multiplexed arrangement require a considerable sacrifice in the speed at which the medium moves past styli of the electrostatic print head.
Electrostatic printing typically uses two voltage sources to effect printing. One source, which can be described as a bias source, produces a voltage difference between the styli and the print medium of a level below that necessary to effect transfer of charge between the styli and the print medium. This bias voltage is generally applied to the media through a conductive back plate arrangement to a conductive layer of the print medium which is well known in the art. Bias voltage can also be applied to the media via capacitative coupling from a set of front plates. Such arrangements are described, for example, in U.S. Pat. No. 3,611,419. It is also possible to place a DC bias voltage on styli directly. In either event, the additional voltage required to effect printing is supplied to the selected styli via an additional electronic pulsing circuit that is controlled in such a way as to produce the desired image on the print medium.
One of the problems of the prior art styli is that with a D.C. bias voltage applied to head styli that is below the voltage at which printing occurs, a spurious discharge occurs from some styli to the print medium which upon subsequent toning causes discoloration of the medium in random ways bearing no relationship to the image desired to be formed on the medium. This "background noise" is undesirable and must be reduced to a very low level if not totally eliminated. To do this, the DC bias voltage must be reduced to a lower level and consequently the pulsing circuitry, that is used in conjunction with the D.C. bias circuitry to provide the required printing voltage to the various styli in the printing head must be made capable of a larger voltage swing than before necessary. This increases cost and complexity of the electronics and shortens the life of the print head due to degradation of the insulative material between styli due to the new higher voltages between adjacent styli. This effect will be explained in detail later.